An electric wheeled vehicle is a vehicle in which the wheels are only driven by an electric motor. Electric vehicles do not emit pollutants, but tend to have a more limited range than vehicles powered by an internal combustion engine (ICE).
Hybrid vehicles are typically powered by both an internal combustion engine (ICE) and an electric motor. Hybrid vehicles provide a longer operating range than purely electric vehicles while emitting fewer pollutants than purely ICE powered vehicles.
In what is commonly known as a series hybrid vehicle, the ICE is used to turn a generator. The generator produces electricity that is used to power the electric motor and/or charge the batteries. The electric motor drives the transmission, which in turn drives the wheels. As such, in a series hybrid vehicle, the ICE is only used to produce electricity.
Another type of hybrid vehicle is what is commonly known as a parallel hybrid vehicle. Typically, in this type of hybrid vehicle, both the ICE and the electric motor drive the wheels. Depending on the operating condition, only the electric motor can drive the wheels, only the ICE can drive the wheels, or both the ICE and the electric motor can drive the wheels together.
Many vehicles that are provided with an electric motor and/or a generator are capable of regenerative braking. Regenerative braking is an energy recovery mechanism which slows a vehicle by converting its kinetic energy to another usable form. In hybrid and electric vehicles, this recovered energy is typically stored within a battery or capacitor by using the vehicle's electric motor as a generator to charge the batteries.
The efficiency of a given regenerative braking system can vary as a function of a number of parameters such as: braking torque, electric motor speed, vehicle speed, and state of charge. In order to maximize energy recuperation, a control unit sets the braking (negative) torque applied by the electric motor during regenerative braking, thereby controlling the rate at which the battery is charged and the rate at which the vehicle decelerates as a result of the braking torque.
An “optimal” braking torque is defined as a braking torque that maximizes energy recovery. This optimal torque, estimated by the control, is typically calculated as a function of vehicle speed. This optimal torque can be used in the two modes described below.
In an automatic mode, the user's input is limited to an instruction to decelerate and the control unit controls the rate of deceleration (in order to maximize regeneration) by setting the braking torque to the calculated optimal torque at each moment during braking.
In a manual mode, the user controls the braking torque (via a brake lever or brake pedal) while information is displayed as to the efficiency of their braking. If desired, the user can adjust their braking request accordingly by increasing or decreasing the braking request to approach the optimal braking torque. This also serves to teach the user to brake efficiently.
Conventional regenerative control systems typically set braking torque (Tb) as a function of vehicle speed (v) via a map KT obtained experimentally or from a function Tb=KT(v). There are at least two inconveniences with this system.
The first inconvenience is that the map KT is obtained for a specific set of operating conditions, such as mass of driver, passenger and baggage; distribution of those weights; road condition (clean with good grip, slippery, bumpy, etc.); environment (head/tail wind); slope of road (uphill/downhill). For example, a conventional map KT may be calibrated for a driver of average mass travelling on flat, dry asphalt with no wind. It is possible to provide multiple maps for different sets of conditions, but it would be prohibitively expensive to provide both the sensors for sensing all these factors and the maps for all possible combinations of those factors. Consequently, the “optimal” torque obtained by the function KT(v) will not be optimal when conditions vary from those assumed by the function KT(v).
The second inconvenience is that variations in these parameters can greatly reduce the critical slip (or critical slip ratio). Slip occurs when the surface of a wheel and the road with which it is in contact are not moving at the same speed. The relative movement between the road surface and the wheel's contact patch, expressed as a percentage, is referred to as a “slip ratio”, commonly referred to simply as “slip”. When braking, a certain amount of slip is normal and even desirable, as it increases the frictional coefficient of the wheel and hence the deceleration. As slip increases from 0% (i.e. no relative movement between the road surface and the contact patch) to a threshold referred to as critical slip, the traction between the wheel and the road will increase. As slip increases beyond the critical slip, which can be between 10% to 15%, traction will decrease. In practice, a vehicle that reaches and crosses the critical slip threshold during a braking event will very rapidly reach 100% slip, i.e. “wheel lock”, unless braking is interrupted by, for example, an anti-lock braking system (ABS). If the optimal braking torque Tb is calibrated assuming a flat, dry asphalt road, but the vehicle is actually travelling downhill, on wet gravel for example, then the output braking torque Tb will be too high, resulting in greater slip and a less efficient regenerative braking compared to what would be obtained under the assumed operating conditions. Moreover, the critical slip on an inclined, wet gravel road will be significantly lower than on the flat, dry asphalt road. As such, the larger than optimal slip which results from the output braking torque Tb may bring the vehicle much closer (or even beyond) its critical slip.
Regardless of whether it is the control unit that sets the braking torque Tb or it is the driver that sets it to the braking torque Tb themselves under guidance from the control unit, setting the braking torque Tb too high is detrimental both to regen efficiency and potentially to safety.
To address the issue of wheel lock, conventional regenerative braking control systems typically rely on ABS. The ABS monitors the rotational speed of each wheel in order to detect wheel slip and intervenes by slowing that wheel (or wheels), thereby reducing slip, before wheel lock-up occurs. U.S. Pat. No. 5,654,887, for example, describes a regenerative braking system that attempts to maximize braking torque by using the ABS to keep the vehicle at the point of critical slip. However, it has been shown that when you factor in both mechanical and electrical losses (aerodynamic forces, rolling resistance, etc.; the efficiency of the electrical system including the motor and batteries), maximizing braking torque does not necessarily maximize the energy recapture. In addition, the ABS typically uses mechanical braking (calipers, brake discs or drums), so any intervention therefrom results in wasted (non-regenerated) energy. Moreover, this approach brings the vehicle closer to instability (wheel lock).
Therefore, there is a desire for a regenerative braking system and method that provides efficient regenerative braking while being less sensitive to changes in the operating conditions of the vehicle.